The invention relates to a temperature indicator for indicating the temperature state of a glass ceramic cooking surface with an indicating device, in whose circuit is provided in a casing, a switching device having an auxiliary heating system optionally constructed as a PTC resistor, a temperature sensor and a switch operable by the latter.
Due to the smooth surface area of glass ceramic cooking surfaces, a problem arises when articles placed thereon by the user are thermo-sensitive. There is also a danger of burning or scalding on coming into contact with the glass ceramic surface. Consequently, there is a need for an indicating mechanism showing that the glass ceramic surface is warm or hot.
An indicating device is already known (DOS No. 2,627,373) in which a temperature sensor is in direct contact with the heating system of the glass ceramic cooking surface and operates a switch for operating an indicating member.
In addition, a residual heat indicating device with a lamp is known, whose circuit contains a switching device comprising a heating resistor and a temperature-dependent switching element (DOS No. 2,808,181). This device is thermally insulated in such a way that the cooling time up to the switching off of the lamp corresponds to the time in which the glass ceramic cooking area drops below a predetermined surface temperature. However, the disadvantages of this device is that the thermal insulation of the casing can only be constructed in such a way that the cooling time from a given temperature up to the time of switching off the lamp represents a constant value, which corresponds to the cooling time of the glass ceramic which is considered to be constant.
However, a problem arises with glass ceramic cooking surfaces in that the cooling period of the surface is not always constant from a given temperature and is instead dependent on which articles are placed on the cooking surfaces. Thus, e.g. if a large pot with boiling oil is located on the cooking surface, the cooling period for the latter is much longer than if no such pot was present. The known indicating device cannot take account of these differences. In addition, in the known device, the switch is exposed to very high temperatures, so a second switch is provided which switches off the auxiliary heating system in the case of elevated temperatures.